The invention relates to a method for coupling radiation emitted by a multitude of LEDs into a fiber-optic light guide, whereby each LED is being arranged stationary, to each LED is assigned a first fiber-optic light guide with first and second coupling surfaces, light is coupled in via a first coupling surface of each of the first fiber-optic light guides from one of the LEDs, and the radiation of the respective activated LED emerging from the second coupling surfaces is transmitted onto a coupling surface of a second fiber-optic light guide. The invention further relates to an arrangement for transmission of radiation emitted by pulse-controlled LEDs via at least one fiber-optic light guide, whereby assigned to each LED is a first fiber-optic light guide with first and second coupling surfaces, each of the first fiber-optic light guides are oriented with their first coupling surfaces towards an associated stationary LED, the second coupling surfaces are arranged on a closed track or are arranged according a grid pattern, and associated with the second coupling surfaces is a coupling surface of a second fiber-optic light guide.
Light sources to be coupled into fiber-optic bundles or optical fibers at present employ either halogen lamps or xenon lamps. Both of these have in common a low efficiency and a correspondingly high power consumption.
Even though white-light LEDs have achieved considerable power levels, their achievable energy density is still significantly lower than that of a xenon lamp, for example. On the one hand this is due to the comparatively large spatial extent of the light-generating surface, and to the wide emission angle on the other, which complicates efficient coupling into fibers or fiber bundles. Even though it is possible to move an optical fiber very close to the chip of the LED or use an imaging optical system, the power density often is not sufficient.
Tapers for the purpose of improving the coupling efficiency often have the disadvantage of further increasing the aperture angle whereas dichroic beam combiners suffer from the drawback of only being useable for combining selective wavelength regions.
For this reason tasks that require high light output that is to be coupled into optical fibers can not be accomplished using LEDs.
US-A-2008/0310181 relates to a high-brightness illumination system to be used in an endoscopic camera. The arrangement comprises groups of blue, green, and red light-emitting diodes, the light of which is transmitted via optical fibers onto a transmission bundle and subsequently is transmitted via a mixer onto an optical bundle 2, in order to subsequently be able to use the light for the endoscopic camera.
The subject matter of U.S. Pat. No. 5,109,447 is a wide-band-signal light source. For this is provided a waveguide coupler, which transmits signals that are transmitted by fiber-optic light guides assigned to a multitude of LEDs to a second fiber-optic light guide, which possesses a wide-band output.
In an illumination device in accordance with US-A-2005/0046807 light from LEDs arranged in a circle is transmitted to a second fiber-optic light guide that is oriented towards an LCD.
DE-U-20 2008 006 191 relates to an arrangement for the use of light-emitting diodes for high-capacity illumination purposes. In this one uses a multitude of light-emitting diodes, whereby to each LED is assigned a first coupling surface of a fiber-optic light guide. The second coupling surfaces of the fiber-optic light guides then are combined in a cable-like fashion in a light-guide unit, in order to form a light-emitting area.
It is the objective of the present invention to further develop a method and an arrangement of the above-mentioned type in a way that allows coupling high light-output power into a fiber-optic light guide by means of LEDs. In this, the aim is to create the option of achieving a power output with any desired amplitude of fluctuations of the luminous intensity, while keeping the constructional complexity low. A further objective of the invention is the minimization of dead times and the simplification of the power supply circuit to the LEDs.
With respect to the method, the objective is essentially met by the LEDs being operated in a pulsed and sequential manner and by the second coupling surfaces being arranged on a closed track, along which is moved the second fiber-optic light guide with its coupling surface or an optical system oriented towards the coupling surface of the stationary second fiber-optic light guide.
The invention utilizes the property of LEDs, particularly of white-light LEDs, that LEDs can be operated in a pulsed manner at power levels that are higher by a factor of 50 or more in comparison to the admissible current during continuous-wave operation. Consequently an LED such as a white-light LED can—when operated in correspondingly short pulses—be operated at a high pulse power while maintaining the average power output for a corresponding duty cycle.
In order to avoid unnecessary dead times and to avoid in particular collector rings to supply power to the LEDs, the invention intends that the LEDs be arranged in a stationary fashion. Associated with the first fiber-optic light guides, which are associated with the LEDs, is a second fiber-optic light guide, which either is stationary or with its coupling surface can be aligned to the second coupling surfaces of the first fiber-optic light guides in a manner so that an alignment is achieved relative to the one fiber-optic light guide, possibly two neighboring fiber-optic light guides, of LEDs that are under power and consequently emit light, in particular to the desired extent and according to the clock rate of the active LEDs.
In other words, one uses any desired number of LEDs, whereby their light is sequentially coupled into first fiber-optic light guides, such as optical fibers or fiber bundles, so that the output power can be increased by a factor of 50 or more.
Another option would be to arrange LEDs on a rotating wheel and to generate a light pulse with an LED at precisely the time when this LED is located opposite to the fiber-optic light guide that is stationary. This however would create unnecessary dead times. The required collector rings for the power supply to the LEDs represent another disadvantage.
The invention's teaching results in the minimization of dead times, the lack of collector rings, and the option of generating an extremely uniform power output or controlled pulses in a defined time pattern. Hereby one also has the option of synchronizing the time base to the frame rate of a recording unit such as a CCD camera.
According to the invention's teaching, the first fiber-optical light guides are arranged in a manner so that to each LED is connected a first fiber-optic light guide, such as a short piece of optical fiber. This can be achieved by directly placing it in proximity of the chip surface of the LED. This creates the possibility of arranging the first fiber-optic light guides in a circular pattern as tightly spaced as possible, whereby the coating of the fiber-optic light guides can be removed at least in the area of the second coupling surfaces, in order to reduce dead times. Associated with the corresponding first fiber-optic light guides, e.g. optical fibers, then is a second fiber-optic wave guide, to transmit the pulsed radiation of the individual fiber-optic light guides. The high-power-density light can for example be used to illuminate an object to be scanned, such as a tooth or a jaw section.
The length of the fiber-optic light guides should be chosen to prevent tight bending radii wherever possible.
One has the option of guiding the second fiber-optic light guide along the second coupling surfaces of the first fiber-optic light guides, whereby this is synchronized to the sequentially activated LEDs. For example, the second fiber-optic light guide with its coupling surface may be moved along a circular path, whereby the second coupling surface of a first fiber-optic light guide of an activated LED at the time of the activation will be centered opposite to the coupling surface of the second fiber-optic light guide.
Instead of a movable second fiber-optical light guide, one also can arrange the latter in a stationary position, whereby for coupling-in the pulsed radiation one uses an optical deviating element, which accordingly is moved along the second coupling surfaces of the first fiber-optic light guides in order to subsequently couple the light into the second fiber-optic light guide. The optical deviating element such as a deviating prism may be arranged on a rotating support in order to redirect the light of the first fiber-optic light guide into the second fiber-optic light guide.
Deviating from the circular pattern, the second coupling surfaces may also be positioned in a grid pattern, to sequentially couple the radiation into the second fiber-optic light guide by means of minors, rotating prism wheels, or combinations of suitable optical coupling means.
When the coupling surface of the second fiber-optic light guide is moving past the second coupling surfaces of the first fiber-optic light guides, the coupling losses are variable and will be dependent on the position of the second fiber-optic light guide moving relative to the first fiber-optic light guides. The same applies for coupling optics that can be moved relative to the first fiber-optic light guides.
To be able to achieve as uniform a power output as possible despite this, the invention intends as a further development that two neighboring LEDs at a time be activated simultaneously. As a result of this, neighboring LEDs are activated when the coupling surface of the second fiber-optic light guide partially optically covers the first fiber-optic light guide associated with the activated LEDs, i.e. is in a position facing them. The remaining ripple can be eliminated by a compensating modulation of the LED current. This may be achieved using either an open-loop or closed-loop control method.
However, a pulsed operation is also possible. In order to achieve maximum coupling efficiency it is possible in the time period just before until just after the optimal coupling point, i.e. when the second coupling surface is directly opposite to the coupling surface of the second fiber-optic light guide, to generate a short light pulse from an LED that may be synchronized with the light integration time of a sensor.
Independently hereof, the invention in particular intends that the second coupling surfaces be arranged on a closed track, along which is moved the second fiber-optic light guide with its coupling surface or an optical system aligned with the coupling surface of the stationary second fiber-optic light guide.
Furthermore, to ensure optimal coupling-in of light, the second coupling surfaces of the first fiber-optic light guides should be aligned parallel relative to the coupling surface of the second fiber-optic light guide.
An arrangement of the above-mentioned type is characterized in particular by the fact that the second fiber-optic light guide can—in accordance with the clocking of the active LEDs—be oriented towards the second coupling surface of a first fiber-optic light guide of at least one active LED, or that an optical deviating element can be aligned with the second coupling surface of a first fiber-optic light guide of an associated activated LED, whereby the optical deviating element directs the radiation onto the coupling surface (44) of the stationary second fiber-optic light guide.
In particular, the LEDs should be arranged on a heat sink serving as a mount, whereby Peltier elements may be present for the elimination of heat.
The number of LEDs may be between 2 and 100.
Preferably the first fiber-optic light guides with their coupling surfaces are arranged on a circular path, along which the coupling surface of the second fiber-optic light guide can be moved, whereby the second coupling surfaces of the first fiber-optic light guides are oriented in parallel to the coupling surface of the second fiber-optic light guide.
For the purpose of reducing dead times, it is intended that the coating of the first fiber-optic light guides has been removed in at least the region of the second coupling surfaces.
If the second fiber-optic light guide is moveable relative to the first fiber-optic light guides, it is suggested that the second fiber-optic light guide be accommodated in a mounting element, which is rotatable about an axis that passes through the mount—such as an annular disk—for the LEDs, whereby the second coupling surfaces preferably are inclined relative to the axis.
For a stationary second fiber-optic light guide, the coupling-in from the first fiber-optic light guides may be achieved via an optical deviating element such as a deviating prism, which is attached to a rotatable mount. The optical deviating element may originate from a rotating disk that is rotatable about an axis, which passes through the LED mount and along which the second fiber-optic light guide extends at least in the area of the optical deviating element.
In order to allow optimal coupling and decoupling it is intended that on the base surface of the deviating prism the second coupling surface of at least one first fiber-optic light guide associated with an activated LED and the coupling surface of the second fiber-optic light guide be aligned, whereby preferably arranged on the base surface are lenses to focus the radiation.